In a corporate (i.e., non-fractional) T1 or E1 digital communications network environment, communication line conductivity and signal/data quality assertion is provided by conventional T1/E1 protocol maintenance signals. These maintenance signals provide assurance against data loss occurring anywhere from the customer's data service unit/channel service unit (DSU/CSU) equipment, continuing through the commercial carrier's central office (CO), the carrier network itself, other connecting central offices of the carriers, and finally through to the customer unit at the destination. Unfortunately, these maintenance signals do not provide the same end-to-end coverage in a fractional T1/E1environment.
In a "fractional" T1/E1 environment, the conventional T1/E1 maintenance signaling techniques will only cover the connection between customer equipment and the carrier network. Consequently, an interruption of service resulting in data loss may go undetected if it occurs within the commercial carrier network. For example, conventional T1/E1 maintenance signals cannot detect the loss of a DS0 channel or an out-of-service condition (OSS) (assuming the integrity of the customer-to-central office connection is maintained). Merely attempting to detect an absence of data is not feasible in a traditional T1/E1 environment because a DS0 time slot in a fractional T1/E1 network is not guaranteed to be a constant when data is not present. One solution has been proposed that entails initiating a Unix-type "ping" messaging arrangement wherein one unit sends a query or test message to another unit which upon receiving the message then reciprocates with a return message indicating that the query was received. A major drawback with this arrangement is that it incurs unavoidable network transmission path delays for each message (i.e., the "ping" must travel through the T1/E1 network twice) in addition to some processing delay at the remote unit. The cumulative delay incurred is often unacceptable for meeting various user requirements. Accordingly, there is a long-felt need for a more efficient signaling arrangement that would allow a user of a fractional T1/E1 digital communications carrier network to protect against a loss of data in the event of a data interruption in the carrier network without incurring unacceptable delays. The present invention solves this problem by utilizing a unique "heartbeat" signalling arrangement that incurs only a single path delay through the T1/E1 network, thus permitting a much faster detection of a loss of communication condition in fractional networks.